Kailash, I never advise anyone for pedigree as it is no good, although I am told pedigree proffessional is a bit better. Never change food suddenly, always do it gradually add a little of new food to old one and increase the quantity every few days till the old food finishes off and the dog is eating the new one only.
Yes Royal Canin is expensive, but I personally like it and for years my dogs were on it, then there came the foot and mouth disease and the supplies stopped in my area so I shifted to Eukanoba and now I am still with it and having no problem, although since you have a puppy I would advise you keeping the pup on it for atleast 6 months and then shifting to Eukanoba. But if you still think it will not suit you then by all means go for the large breed puppy in Eukanoba.
What you can do is also try and buy your food from one store or vet, so that he can then may be offer you a discount. Since I generally used to buy quite a bit the company gave me a special discount, but even your vet if he sells dog food can give you upto 15 % off.
Some people even give half pedigree and half Eukanoba or RC, but what you need to see is that during the pups growing age it should get a good wholesome diet and also required calcium and multi vitamins in the right quantity.